All hail: by 2070, icy pellets hitting state's mountain flanks may be a thing of the past

If you are college-age or younger, you might just live to see the day when hail disappears from the eastern flanks of Colorado’s Rocky Mountains.

A new modeling study involving the Cooperative Institute for Research in Environmental Sciences, a joint institute of the University of Colorado Boulder and the National Oceanic and Atmospheric Administration, indicates hail will likely cease to fall in those locales by the year 2070, a result of rising temperatures.

Less hail damage could be good news for gardeners and farmers, said lead author Kelly Mahoney, a CIRES research scientist, but a shift from hail to rain can also mean more runoff, which could raise the risk of flash floods. “In this region of elevated terrain, hail may lessen the risk of flooding because it takes awhile to melt,” Mahoney said. “Decision makers may not want to count on that in the future.”

For the study, published in the journal Nature Climate Change, Mahoney and her colleagues used “downscaling” techniques to try to understand how climate change might affect hail-producing weather patterns across Colorado.

The research focused on storms involving pea-sized and smaller hailstones on Colorado’s Front Range, a region that stretches from the foothill communities of Colorado Springs, Denver and Fort Collins up to the Continental Divide. Colorado’s most damaging hailstorms tend to occur further east and involve larger hailstones not examined in this study.

In the summer on Colorado’s Front Range above about 7,500 feet, precipitation commonly falls as hail. Decision makers concerned about the safety of mountain dams and flood risk have been interested in how climate change may affect the amount and nature of precipitation in the region.

Mahoney and her colleagues began exploring that question with results from two climate models, which assumed that levels of climate-warming greenhouse gases will continue to increase in the future, from about 390 parts per million in the atmosphere today to about 620 parts per million in 2070.

But the weather processes that form hail, like thunderstorms, occur on much smaller scales than can be reproduced by global climate models. So the team “downscaled” the global model results twice: first to regional-scale models that can take regional topography and other details into account, then again to weather-scale models that can resolve individual storms and even the cloud processes that create hail.

Finally, the team compared the hailstorms of the future, from 2041 to 2070, to those of the past, from 1971 to 2000, as captured by the same sets of downscaled models. Results were similar in experiments with both climate models. “We found a near elimination of hail at the surface,” Mahoney said.

In the future, increasingly intense storms may actually produce more hail inside clouds, the team found. However, because those relatively small hailstones fall through a warmer atmosphere, they melt quickly, falling as rain at the surface or evaporating back into the atmosphere. In some regions, simulated hail fell through an additional 1,500 feet of above-freezing air in the future, compared to the past.

Co-authors of the study include James Scott and Joseph Barsugli of CIRES and NOAA, Michael Alexander of the NOAA Earth System Research Laboratory and Gregory Thompson of the National Center for Atmospheric Research.